1. Field of the Invention
The present invention relates generally to an automatic noise eliminating device incorporated into an FM receiver, and more specifically to a device incorporated into an FM receiver for eliminating higher frequency noise from an FM audio signal generated by multipath transmission.
2. Description of the Prior Art
The field strength of an electric wave of frequency approximately 100 MHz used for FM broadcasting purposes becomes weaker as the distance from the transmitting station increases. However, if the FM receiving station is located on top of a hill, for example, the field strength will be greater than if the receiving station is located at the same distance, but in a valley or surrounded by high buildings.
Consequently, the field strength depends on the environment of the FM receiving station and the signal to noise ratio of the FM receiver (hereinafter abbreviated as S/N ratio) changes according to the field strength.
Conventional FM receivers may be provided with noise eliminating circuits for compensating for the changes in the S/N ratio.
A brief description will be made of a conventional FM receiver.
The FM intermediate frequency signal IF fed from a high frequency amplifier circuit and a frequency mixing circuit, etc., is amplified by an FM intermediate frequency amplifier, by an amplitude limiter, demodulated by an FM detector, and, in a multiplex system, separated by a stereo decoder into signals for two channels; left L and right R. The demodulated output signal is fed into a deemphasis circuit, where the gain of the high frequency components is lowered to return the overemphasized high frequency components in a preemphasis circuit of an FM transmitter to the original gain.
These two-channel signals are finally amplified by audio frequency amplifiers, respectively.
At the same time, the FM intermediate frequency signal IF' produced by the FM intermediate frequency amplifier is demodulated by an AM detector to produce an envelope signal depending on the amplitude variation (of frequency roughly 20 Hz to 100 Hz) corresponding to the changes in field strength appearing for the reason described above.
This signal is fed into a control circuit, which produces a control voltage according to the input signal level. The control voltage is applied to three attenuators to control the amount of attenuation. The attenuators comprise, e.g., voltage-resistance converters. One attenuator is connected from ground to each output terminal of the stereo decoder via a capacitor allowing only high frequency signals to pass. The third attenuator is connected across the two output terminals of the stereo decoder via a high frequency bypass stereo separation capacitor.
When the field strength becomes weak and the output FM audio signal is low, the high frequency region of the FM audio output signal from the stereo decoder is attenuated and the stereo separation degree is reduced, in order to improve the S/N ratio of the FM receiver.
However, such conventional FM receivers have a shortcoming; since the coupling factors of the three capacitors are changed in response to the field strength variation in the relatively low frequency region, in order to reduce noise generated as described above, the amplitude modulated signal noise (1 to 50 KHz) of the carrier of the directly received wave caused by reflected signals is not filtered out, so that such noise due to multipath transmission cannot be eliminated.
Noise due to multipath transmission has a particularly serious influence on an FM receiver mounted in an automotive vehicle, which while moving about remarkably changing environment is often subjected to multipath transmission.